The flu is caused by either the influenza A virus or the influenza B virus. According to the most recent seasonal statistics from the Centers for Disease Control and Prevention (CDC), about 30 percent of people diagnosed with the flu by lab tests have influenza B and 70 percent have influenza A.

The signs and symptoms of influenza A and B are very similar, but influenza B symptoms are often less severe. As well, some symptoms tend to be more common with influenza B than with influenza A. But despite these slight differences, influenza A and B are treated and prevented in the same ways.

Influenza B General Signs and Symptoms

Influenza B, like influenza A, typically begins with the sudden onset of a fever. The temperature usually reaches 39 degrees Fahrenheit or higher. It tends to be higher in children than in adults. As a person’s temperature is rising, chills and shivering may occur. Sweating can appear once the fever has set in. The CDC notes that the fever usually lasts for about 3 to 4 days.

Soon after the fever begins, other generalized symptoms often appear. These include a headache, muscle aches, other body aches and tiredness.

Respiratory Signs and Symptoms

What Is the Difference Between Influenza A & B?

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Respiratory signs and symptoms are also common with the flu. A cough is the most frequent respiratory symptom, which can be severe. A stuffy or runny nose, sore throat and sneezing may also occur, but they are usually relatively minor -- less frequent and less severe than with a cold. A runny nose tends to be more common with influenza B than with influenza A, according to the medical textbook “Influenza.” Respiratory symptoms usually disappear within 1 to 2 weeks, but an intermittent cough may remain for several weeks.

Digestive Tract Signs and Symptoms

Poor appetite is common with the flu. Nausea, vomiting and abdominal pain may occur as well, especially in children. Digestive tract symptoms are also more common with influenza B than with influenza A, according to “Influenza.”

About Prolonged Flu Symptoms

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If you think you may have the influenza B, see your doctor promptly. Your doctor may recommend an antiviral medication, but to be effective, these medications must be started in the first 48 hours after the onset of symptoms. Other treatments include over-the-counter medications to reduce your fever, resting and staying hydrated.

Also seek prompt medical care if your symptoms do not begin to improve in a few days or if they were improving but are now becoming worse. Obtain immediate medical care any time you have shortness of breath, chest pain, dizziness or lightheadedness, confusion or severe or persistent vomiting.

A yearly flu vaccine will reduce your chances of getting influenza B. The vaccine acts against influenza B, as well as influenza A. Even if it does not prevent you from becoming sick, the vaccine will generally make the infection less severe and help you recover more quickly. To avoid spreading the flu, the CDC recommends staying home for at least 24 hours after your fever is gone, except to obtain medical care.

Reviewed and revised by Mary D. Daley, MD